In most conventional zoom lens barrels, such as those used for cameras, one driving motor is used to move a plurality of lenses to change a focal length during a zooming operation, and another driving motor is used to move a part of the lenses to adjust the focus during a focusing operation. In order to manufacture a compact camera at a low cost, it is preferable to use only one motor to perform both the zooming and focusing operations, and some kinds of zoom lens barrels, which allow zooming and focusing operations to be accomplished by only one motor have been proposed.
Referring to FIG. 9, illustrating a lens barrel 1a, which is disclosed in Japanese Laid-Open Patent Publication No. 6-202213, a driving motor 6 and a rotation-transmission gear train 7 are provided inside the lens barrel 1a. The rotation-transmission gear train 7 serves as a mechanism for switching the rotation of the driving motor to transmit the driving force thereof to either a zooming mechanism or a focusing mechanism. The lens barrel 1a is required to be of a large size because it is necessary to accommodate the driving motor 6 and the rotation-transmission gear train 7 inside the lens barrel 1a.
On the other hand, the lens barrel 1b illustrated in FIG. 10 which is disclosed in Japanese Laid-Open Patent Publication No. 4-317015 is comparatively of a small size because a driving motor 6a is positioned outside the lens barrel 1b. The lens barrel 1b has a first driving ring 2 and a second driving ring 4 coaxially connected with each other in series with an idling range or a play which is provided therebetween. The first driving ring 2 has a cam groove 2a for moving move a first lens group 8, and the second driving ring 4 has a cam groove 4a for moving the second lens group 9, respectively. In the lens barrel 1b, a focusing operation is performed within the idling range in which the connection portion 2b of the driving ring 2 and the connection portion 4b of the driving ring 4 are not in contact with each other. That is, when the connection portion 2b of the driving ring 2 and the connection portion 4b of the driving ring 4 are within the idling range, the rotation of the driving motor 6a is transmitted to only the second driving ring 4, so as to move the second lens group 9, with the first driving ring 2 and the second lens group 8 remaining stationary.
When the second driving ring 4 is rotated more than a predetermined amount, and the connection portions 2b and 4b contact each other, i.e. when the first and second driving rings 2 and 4 are out of the idling range, the rotation of the driving motor 6 is transmitted from the second driving ring 4 to the first driving ring 2. Thus, the zooming operation is performed so that both lens groups 8 and 9 move along the cam grooves 2a and 4a at the same time, respectively.
In other words, according to this latter conventional mechanism, the focusing operation is performed when the first and second driving rings 2 and 4 are within the idling range, whereas the zooming operation is performed when the first and second driving rings 2 and 4 are out of the idling range.
According to this latter conventional mechanism, because the second lens group 9 moves along the cam groove 4a to perform both the focusing and zooming operations, the resolution and accuracy of the focusing operation are the same as the resolution and accuracy of the zooming operation.
As shown in FIG. 11, however, the movement amount of the second lens group 9 during the focusing operation is much smaller than that of the first and second lens groups 8 and 9 during the zooming operation. For example, the movement amount of the second lens group 9 during the focusing operation is less than 1/3 of that of the first and second lens groups 8 and 9 during the zooming operation.
Generally, in order to realize a higher resolution during focusing, it is necessary to realize a higher accuracy during the focusing operation. However, if the cams are formed so as to ensure a predetermined amount of movement of the first and second lens groups 8 and 9 during the zooming operation, the shapes or configurations of the cams must be steep, which makes achieving higher resolution and accuracy in the focusing operation difficult.